1. Field of the Invention
The present invention relates to a sensor device for detecting a specified chemical component in a solution and, more particularly, to such a sensor device having a reference FET which exhibits a constant potential with respect to a solution without responding to the chemical components such as ions in the solution.
2. Description of the Related Art
Ion-sensitive electrodes (ISEs) have been widely used for detecting the ion concentration of a solution. Recently demands for miniature sensors capable of detecting the concentration of a specified ion species in a very small region are on the increase in the field of, for example, clinical medicine. In an ISE using, for example, a glass electrode, the glass film inherently has a high resistance as high as about 10 M.OMEGA.. In order to use such an electrode type sensor in detecting the components of a solution, it is necessary to use an amplifier having a high input impedance. Also, where the sensor of this type is miniaturized, the resistance of the glass film is increased. It follows that a miniature electrode type sensor tends to invite an external noise, leading to a delay in the response speed. Under the circumstances, an ion sensitive FET (ISFET) in which an ion sensitive film is formed on the gate region of an insulated-gate FET is getting attentions nowadays. Since the ISFET exhibits an impedance converting function, it permits the miniaturization, making it possible to overcome the above-noted problems inherent in the electrode type sensor.
In the case of using an ISFET, it is also necessary to use a reference electrode which provides a basis of the relative potential value. Naturally, it is absolutely necessary to miniaturize the reference electrode in the case of miniaturizing a sensor using an ISFET. A reference electrode of a half cell type, in which a single electrode such as a calomel electrode or silver/silver chloride electrode is in contact with an electrolyte solution, is known in the art. However, the electrode of such a half cell type unavoidably necessitates a liquid junction, with the result that, when the electrode is miniaturized, the resistance is increased in the liquid junction, leading to an unstable output potential. In addition, the electrode of a half cell type tends to give rise to leakage of the electrolyte solution from the liquid junction portion. The leakage of the electrolyte solution results in difficulties such as an increased obstructive ion concentration and an increased ionic strength in the step of detecting the ion species in a very small region, making it impossible to detect ion concentration accurately. Further, it is impossible to markedy diminish the amount of the electrolyte solution in view of the leakage. This also gives a restriction to miniaturization of the reference electrode.
Under the circumstances, a FET type reference electrode (reference FET), in which the gate region of an insulated-gate FET is covered with a non-porous film formed of a hydrophobic polymer is described in Published Examined Japanese Patent Application No. 58-25221. The reference FET of this type utilizes the fact that since ion dissociating groups such as hydroxyl groups are not present on the surface of the hydrophobic polymer film formed on the gate insulation film, the polymer film does not respond to specified ion specing, giving a constant gate potential to the reference FET. When the ion concentration of a solution is actually detected by using the reference electrode of this type, an ISFET and an inactive metal electrode serving to give a predetermined potential to a solution are dipped in the solution together with the reference electrode so as to measure the potential difference between the ISFET and the reference FET.
It has been found, however, that not all the hydrophobic polymer film are suitable for use as an insensitive film of a reference FET. For example, hydrophobic polymer films insensitive to specified ion species but sensitive to electrostatic properties of the solution such as the ionic strength are not suitable for use as an insensitive film of the reference FET. If the hydrophobic polymer film is sensitive to the ionic strength of a solution, the potential of the reference FET is not maintained constant. It follows that a sensor device using such a hydrophobic polymer film is incapable of accurately detecting the concentration of the specified ions.
As described above, the prior art reference FET is disadvantageous in that the it is affected by, in particular, the ionic strength of the solution, failing to maintain a constant potential.